A University of Alberta team has developed a new way to synthesize uniform, well-characterized nanocrystals of inexpensive zinc phosphide (Zn3P2). These semiconducting ‘quantum dots’ could lead to cheap, printable photovoltaic cells.

Due to their small size, semiconducting nanocrystals can exploit the effects of quantum physics to turn solar radiation into electricity in an entirely different way from traditional silicon-based photovoltaics. But many of the materials currently used to make quantum dots — lead sulphide or cadmium telluride for example — are made from precursors that are toxic and/or not earth-abundant. “If you’re looking for materials that have good photoelectrical properties, zinc phosphide is one of them,” says Jillian Buriak, professor of chemistry at U of A. 

Nanoparticles of zinc phosphide have been created before but they were chunky and not very uniform. In a paper in ACS Nano, Buriak’s team describes a new solution-based process that uses precursor molecules of dimethyl zinc and tri-octylphosphine. “It wasn’t easy to begin with but now anyone can do it,” says Buriak. Transmission electron microscopy and X-ray diffraction showed the quantum dots to be perfectly uniform, with a diameter of approximately eight nanometres. 

The team made the quantum dots into solar devices that were very light sensitive, but unfortunately produced limited current. The likely culprit is a layer of elemental phosphorus that forms as the P moves from a +3 oxidation state in the trioctylphosphine to a -3 state during formation of Zn3P2. The team has since worked out a new synthesis method as well as improved device designs. “If you can make this material as a solution-processable ink, you could make spray-on solar cells,” says Buriak. “Then you’d really have something cool.”